Tin oxide finds use in the semiconductor field because of its transparent and semiconductive properties. For instance, it is superior to polysilicon or other conductors for applications such as CCD imager electrodes because of its transparency.
Tin oxide however has a disadvantage in that it is extremely refractory in nature, and is highly resistant or impervious to most etchant materials. As disclosed in German Pat. No. 2,425,702, it is possible to etch tin oxide using a hydrogen iodide 45% to 51% aqueous solution if the solution is heated to approximately 90.degree. C. This method has been applied to semiconductor structures, but the results have been unsatisfactory. When etched by this wet etch, layers of tin oxide showed cracking and a differential attack along the grain boundaries of the tin oxide film. As will be explained below, other gas etchants have been found to be anisotropic, that is, they do not etch tin oxide at an equal rate in all directions. In cases of complex surface topography, this has led to undesirable tin oxide remnants that in turn cause shorts between circuit components.
There therefore exists a need in the industry to provide a method and apparatus for isotropically etching tin oxide on CCD imagers and other integrated circuits where high pattern resolution is necessary.